A seamless steel tube/pipe is produced by the Mannesmann tube-making process. The Mannesmann tube-making process includes the following steps:
(1) piercing-rolling a starting material (round billet) heated at a predetermined temperature into a hollow shell by using a piercer;
(2) elongation-rolling the hollow shell by an elongation rolling mill (e.g. mandrel mill); and
(3) carrying out diameter adjusting rolling on the elongation-rolled hollow shell to have a predetermined outer diameter and wall thickness by using a diameter adjusting rolling mill (e.g. a stretch reducer).
In the piercing-rolling by using the piercer, a plug is used as a piercing tool. This plug is mounted to a front end of a core bar so as to pierce a billet heated at a high temperature of approximately 1200° C.; thus the plug is subjected to a hostile environment with a high surfacial pressure and a high temperature. In general, the plug includes a base metal made of hot working tool steel, and a film of oxide scale is formed on a surface of the base metal through a heating process in advance for the purpose of protection of the base metal, and thereafter the plug is used in the piercing-rolling. During the piercing-rolling, the scale film on the surface of the plug insulates heat transfer from the billet to the base metal of the plug, and also prevents seizing between the billet and the plug.
Repetitive piercing-rolling of such a plug having the scale film causes a gradual abrasion of the scale film. The abrasion of the scale film deteriorates thermal insulation effect of the film, resulting in increase in temperature of the plug during the piercing, so that melting-incurred metal loss and deformation by heat are likely to be caused to the plug base metal. If the scale film is exhausted, and the plug base metal comes into direct contact with the billet, seizing is caused, so as to generate flaws on an internal surface of a steel tube/pipe. Consequently, the plug becomes unusable at the moment when the film is exhausted, and its durability life is expired.
Particularly in production of a seamless steel tube/pipe made of high alloy steel such as high Cr steel containing Cr of 9% or more, Ni-based alloy, and stainless steel, significant abrasion of the scale film on the surface of the plug is generated during the piercing-rolling, so that the durability life of the plug becomes significantly reduced. For example, in the case of piercing stainless steel, the scale film on the surface of the plug becomes worn away through two or three passes (the number of times of continuous piercing rolling), and the durability life of this plug is expired. This requires a frequent replacement of the plug, which deteriorates the production efficiency. In production of a seamless steel tube/pipe of high alloy steel, it is required to enhance the durability life of the plug during the piercing-rolling, thereby enhancing the production efficiency of the steel tube/pipe.
To satisfy such a requirement, as an example of the film formed on the surface of the plug base metal, Patent Literature 1 discloses such a plug having a film containing oxide and Fe formed on the surface of the plug base metal by performing arc-spraying of iron wires, instead of using the scale film formed through heat treatment. Since the plug having the arc-sprayed film has a film containing oxide and Fe on the surface of the plug, this plug is excellent in thermal insulation performance and seizing prevention, so that enhancement of the durability life of the plug is likely to be achieved.
Patent Literature 1 discloses an equipment system of producing (reproducing) a plug having an arc-sprayed film by forming the film containing oxide and Fe on a surface of a base metal of the plug in such a manner that, after shotblasting is applied onto the surface of the plug, molten material is sprayed from arc-spray guns onto the surface of the plug base metal while a turntable on which the plug is mounted is being rotated. In this equipment system, the spray guns are so disposed as to face a tip end portion, a front-half of the body portion, and a rear-half of the body portion of the surface of the plug base metal, and forms the arc-sprayed film by operating all the spray guns at the same time, thereby reducing time required for forming the film compared to the case of using a single spray gun to form the arc-sprayed film across the entire surface of the plug base metal, which results in enhancement of production efficiency of the plug.
Unfortunately, even in the plug having the arc-sprayed film formed by using the conventional equipment system disclosed in Patent Literature 1, there occurs separation of the film if a billet length to be pierced is long, or if a billet having elevated-temperature strength is used. As such, there is still room for further improvement in securing the steadily enhanced durability life of the plug, and thus it has been strongly desired to produce a piercing-rolling plug that can realize the above improvement.